Greenhouse Fan and Pad Evaporative Cooling
Sizes pad area and airflow, then computes the pad-outlet temperature and the rise to the fan end.
Example
You enter
- Floor area (sq ft) 2880
- Airflow per square foot (cfm) 8
- Pad face velocity (fpm) 250
- Pad height (ft) 5
- Outdoor dry bulb (deg F) 95
- Outdoor wet bulb (deg F) 75
- Pad saturation efficiency (%) 85
- Solar gain through glazing (Btu/h per sq ft) 188
- Crop latent fraction 0.5
You get
- Total airflow 23040 cfm
- Pad face area 92.16
- Pad length (ft) 18.432
- Air leaving the pad 78
- Cropped sensible (Btu/h) 270720
- Rise to the fan end 10.9 deg F absorbing 270720 Btu/h
- Air at the fans 88.8796
- With no crop transpiring 21.7593
- Empty house fan end f 99.7593
Details, formula, and sources
The worked house runs 78 degF at the pad and 89 degF at the fans; empty it and the rise doubles, because a transpiring crop is part of the cooling system rather than a load on it.
pad outlet = dry bulb - saturation efficiency x (dry bulb - wet bulb); pad area = airflow / face velocity; pad-to-fan rise = sensible gain / (1.08 x airflow).
Customary greenhouse figures: 8 cfm per square foot of floor in summer and 250 fpm pad face velocity for cellulose. Pad manufacturer saturation data and the design wet bulb for the site govern.
Public psychrometric relations; saturation efficiency comes from the pad manufacturer.
Estimate. AHJ and licensed professional govern.
Field names used by the API: floor_area_sqft, airflow_per_sqft_cfm, pad_face_velocity_fpm, pad_height_ft, outdoor_dry_bulb_f, outdoor_wet_bulb_f, pad_efficiency_pct, solar_gain_btuh_per_sqft, latent_fraction, total_airflow_cfm, pad_area_sqft, pad_length_ft, pad_outlet_temp_f, cropped_sensible_btuh, temp_rise_f, fan_end_temp_f, empty_house_rise_f, empty_house_fan_end_f
- Gradient the crop gets a gradient, not pad-end air -- a uniform crop grown in two climatesspec-v1751 method
- Latent fraction entered; it varies with crop, stage, and how well watered the crop isgrower observation